1. Field of the Invention
The present invention relates to circuit board surface structures, and more particularly, to a circuit board surface structure comprising a circuit board with electrically connecting pads having conductive elements formed thereon for external electrical connection.
2. Description of the Prior Art
According to flip chip technology nowadays, a semiconductor chip is electrically connected to a circuit board, in which electrode pads are provided on the active surface of the integrated circuit-equipped (IC-equipped) semiconductor chip, and the circuit board is provided with electrically connecting pads corresponding to the electrode pads. A solder structure or other conductive adhesive material is formed between the electrode pad of the semiconductor chip and the electrically connecting pad of the circuit board for providing electrical connection and mechanical coupling between the semiconductor chip and the circuit board.
As shown in FIG. 1, in the flip chip technology, a plurality of metallic bumps 11 are formed on the electrode pads 12 of a semiconductor chip and a plurality of pre-solder structures 14 made of solder materials are formed on the electrically connecting pads 15 of a circuit board 16, then after a reflow soldering process, the pre-solder structures 14 are reflow soldered to the corresponding metallic bumps 11, so as to form a plurality of solder points 17. Subsequently, an underfill material 18 is provided between the semiconductor chip 13 and circuit board 16, coupling the semiconductor chip 13 with the circuit board 16, so as to ensure a satisfactory electrical connection.
Referring to FIGS. 2A-2D, cross-sectional views illustrating the steps of depositing a solder material on electrically connecting pads of a circuit board are shown. As shown in FIG. 2A, a circuit board 20 having electrically connecting pads 201 on a surface thereof is provided, whereon a solder resisting layer such as solder mask layer 21 is applied on the surface of the circuit board 20. A photomask 22 is formed above the electrically connecting pads 201, and through a series of exposure and development processes, openings are formed to expose the electrically connecting pads 201. In the exposure process, light rays which might otherwise travel straight on bend beneath the opaque region 22a of the photomask 22 due to diffraction, as indicated by the direction S, such that part of the solder mask layer 21 under the photomask 22 is illuminated. Next, as shown in FIG. 2B, after the development process, the opening 210 is tapered downward. Subsequently, as shown in FIG. 2C, a stencil board 23 having a plurality of grids 23a is disposed on the circuit board 20, in which each grid 23a of the stencil board 23 is corresponding to each electrically connecting pad. A solder material is applied on the surface of the stencil board 23, followed by using a roller 24 to roll back and forth on the stencil board 23, or spraying the solder material (not shown) in each grid 23a of the stencil board 23 on each electrically connecting pad 201 after removal of the stencil board 23. As shown in FIG. 2D, then a reflow soldering process is performed to reflow solder the solder material to form solder bumps 25 on the electrically connecting pads 201 of the circuit board 20, such that solder structures can be successfully formed on the circuit board using a stencil printing technology.
However, in the demand of miniaturization and increased functionality of electronic products, the circuit layout is designed to be higher in density and the distance between each circuit layer is also shortened. Thus, a circuit board with high density of circuit layout must have a reduced circuit width and smaller electrically connecting pads. As a gap between the circuits is reduced, the opening on the solder mask layer above the electrically connecting pads must be miniaturized further, and this may reduce the contact area between the solder structure and electrically connecting pad, such that it is more difficult to form a solder material on the electrically connecting pad, thereby failing to satisfy the requirement for fine pitch of the electrically connecting pads in advanced electronic products.
Moreover, as the opening 210 of the solder mask layer 21 is have a tapered downward shape, the bottom portion of the solder bump 25 is smaller than the top portion thereof, causing a decrease in the contact area between the solder bump 25 and the electrically connecting pad 201, thereby reducing the bonding between the solder bump 25 and the electrically connecting pad 201. In addition, the solder bump 25 in the opening 210 of the solder mask layer 21 is also tapered downward, but is not provided with any other coupling means, and thus the solder bump 25 is likely to detach from the electrically connecting pad 201.
Accordingly, there is an urgent need for developing an electrical connection structure for circuit boards with a view to overcoming drawbacks of the prior art, namely difficulty in forming a solder structure on surfaces of electrically connecting pads, difficulty in meeting the requirements for a fine pitch between electrically connecting pads of advanced electronic products, and detachment of conductive elements.